How to redefine find() method for ActiveRecord::Base?


#1

Hi Railers.

I am trying to add very simple caching system for ActiveRecord to my
application and I need advice from Ruby (or RoR) guru.
Everything that I need is redefine find method for some of my models. I
am
trying to do with folowing code

class ActiveRecord::Base
def self.cacheable
class_eval <<-end_eval
alias old__find find

  def self.find(*args)
    puts "We are in find"
  end
end_eval

end
end

Then I adding cacheable to my model. But when I run test I have
following
error

C:/work/agora/agora/config/…/lib/model_cachable.rb:3:in module_eval': (eval):2:inmodule_eval’: undefined method find' for classCategory’
(NameError)
from C:/work/agora/agora/config/…/lib/model_cachable.rb:3:in
module_eval' from C:/work/agora/agora/config/../lib/model_cachable.rb:3:incacheable’
from C:/work/agora/agora/config/…/app/models/category.rb:2
from
c:/progra~1/ruby/lib/ruby/gems/1.8/gems/activesupport-1.2.3/lib/active_support/dependencies.rb:207:in
load' from c:/progra~1/ruby/lib/ruby/gems/1.8/gems/activesupport-1.2.3/lib/active_support/dependencies.rb:207:inload’
from
c:/progra~1/ruby/lib/ruby/gems/1.8/gems/activesupport-1.2.3/lib/active_support/dependencies.rb:39:in
require_or_load' from c:/progra~1/ruby/lib/ruby/gems/1.8/gems/activesupport-1.2.3/lib/active_support/dependencies.rb:22:independ_on’
from
c:/progra~1/ruby/lib/ruby/gems/1.8/gems/activesupport-1.2.3/lib/active_support/dependencies.rb:30:in
`associate_with’
… 35 levels…
from C:/work/agora/agora/test/unit/…/test_helper.rb:3

Seems that alias could not find function find(). Where it defined?? How
could I do alias for it?? Probably there is other simplier way to
overwrite
find() method??


#2

In development mode, Rails reevaluates all your model classes. So your
first
call to cacheable method works, but the second gives you this exception
since
it lost the reference to the original AR::Base.find method.

Try this

class ActiveRecord::Base
def self.cacheable
return if singleton_methods.include? ‘old__find’
class << self
alias old__find find
def find
puts “We are in find”
end
end
end
end

Kent.


#3

Seems that alias could not find function find(). Where it defined?? How
could I do alias for it?? Probably there is other simplier way to overwrite
find() method??

There is no clean way to override find(). Due to the way it takes
multiple types of parameters, it ends up calling itself. To counter
this, I actually added a new option:

def find(args)
options = extract_options_from_args!(args)
call_original_find = lambda { original_find(
(args << options)) }

if !options[:with_deleted]
with_deleted_scope { return call_original_find.call }
end

call_original_find.call
end

def find_with_deleted(args)
original_find(
(args <<
extract_options_from_args!(args).merge(:with_deleted => true)))
end

The whole plugin can be viewed at
http://techno-weenie.net/svn/projects/plugins/acts_as_paranoid/lib/acts_as_paranoid.rb

As the author of a plugin that overrides find, I would suggest not
doing it. It’s way more trouble than it’s worth.

KIDS: Don’t try this at home


rick
http://techno-weenie.net


#4

In development mode, Rails reevaluates all your model classes. So your
first
call to cacheable method works, but the second gives you this exception
since
it lost the reference to the original AR::Base.find method.

Try this

class ActiveRecord::Base
def self.cacheable
return if singleton_methods.include? ‘old__find’
class << self
alias old__find find
def find
puts “We are in find”
end
end
end
end

Kent.


#5

Thanks everyone for the help.

I redefine find() method but seems ActiveRecord does not intended for
it. It
is strange because there could be a lot of plugins that would require
some
proxying between user calls and db operations. act_as_paranoid one of
them.

The problem is that find() recursively calls find(). (See AR code). So
when
I call for example MyModel.find(3) it first calls my version of find, I
am
doing caching operations and then call original method (from AR). AR
adds
some params and also calls find() which is MINE version, and again I do
some
cache operation and call original find(). In my case it works (without
any
infinite recursion) but for complex cases it would be painful, I
believe.

One of the solutions could be adding to Ruby something like self() or
this()
which calls the same method, do recursion. But it would be problematic
to
ask Matz about it, I guess.

Anyway there is very simple cache code that just adds cache for calls
like
MyModel.find(1) and MyModel.find(“434”). It works for me.

class ActiveRecord::Base
def self.cacheable
return if singleton_methods.include? ‘__old__find’

module_eval <<-end_eval
  mattr_reader :cache
  @@cache = {}

  mattr_accessor :cache_hits
  @@cache_hits = 0

  def self.flush_cache
    @@cache = {}
    @@cache_hits = 0
  end
end_eval

class << self
  alias_method :__old__find, :find

  def find(*args)
    #Search only if there is only argument that number or string

representing number
if args.size == 1
arg1 = args[0]
id =
case arg1
when Integer
arg1
when String
arg1.to_i
else
nil
end

      if id and id != 0
        if self.cache[id]
          self.cache_hits += 1
          return self.cache[id]
        end

        #If we did not find it in cache try to select from db
        obj = __old__find(*args)
        if obj
          return self.cache[id] = obj
        end
      end
    end
    return __old__find(*args)
  end
end

end
end